U.S. President Donald Trump launched a fresh attack on NATO allies' failure to raise defense spending on Thursday, prompting leaders to huddle in a special session excluding other summit participants, sources told Reuters.

At one point, in a break with diplomatic protocol, a source said Trump addressed German Chancellor Angela Merkel by her first name and told her: "Angela, you need to do something about this."

"We had a very intense summit," Merkel herself told reporters after the meeting. Trump also prepared to make a previously unscheduled public statement, the White House said.

Invited leaders from non-NATO countries Afghanistan and Georgia were asked to leave along with most NATO leaders' retinues of officials, as the heads of state and government of the Western alliance sought to deal with the man whose nation commands much of the budget and forces for Europe's defense.

Trump had opened the first day of talks in Brussels on Wednesday with a public diatribe against Germany, the second biggest state in the Western defense alliance, before the mood appeared to have calmed as the summit went into its second day, focusing on operations beyond Europe.

But, several sources said, Trump instead reopened in strong terms his demand that other countries greatly speed up their progress toward a NATO target of spending at least two percent of their GDP on defence, which now has a deadline of 2024 with get-out terms available that can stretch it to 2030.

"The language was much tougher today," one source told Reuters. "His harshest words were directed at Germany, including by calling her Angela --'You, Angela.'"

As well as Merkel, Spanish Prime Minister Pedro Sanchez and Charles Michel, the prime minister of Belgium, were also singled out by Trump for undershooting on their spending targets when U.S. taxpayers, funding a defense budget worth about 3.6 percent of their national income, foot much of NATO's bills.

Breaking from a carefully scripted session that was to focus on Ukraine and Georgia, one source said Trump "forcibly restated his position on wanting NATO members to reach 2 percent spending target to a short a deadline."

Two NATO sources said, however, that Trump had not issued a threat to pull the United States out of the alliance that it helped found to keep the peace in Europe after World War Two.

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